U.S. patent number 7,837,680 [Application Number 11/537,396] was granted by the patent office on 2010-11-23 for tuned return electrode with matching inductor.
This patent grant is currently assigned to Megadyne Medical Products, Inc.. Invention is credited to Paul R. Borgmeier, James D. Isaacson.
United States Patent |
7,837,680 |
Isaacson , et al. |
November 23, 2010 |
Tuned return electrode with matching inductor
Abstract
An electrosurgical return electrode for use in electrosurgery.
The return electrode is self-limiting and self-regulating as to
current temperature and temperature rise so as to prevent patient
trauma. According to one aspect of the invention, an inductor is
coupled in series with the electrosurgical return electrode. The
inductor is configured to optimize the flow of the electrosurgical
current by minimizing the effective bulk impedance of the
electrosurgical return electrode when the amount of contact area
between the patient and the electrosurgical return electrode is
sufficient to conduct electrosurgery. According to another aspect
of the present invention, a conductor member is adapted for use
with circuitry that indicates to a user when the contact area
between the patient and the self-limiting member and/or return
electrode is below a given threshold.
Inventors: |
Isaacson; James D. (Salt Lake
City, UT), Borgmeier; Paul R. (Salt Lake City, UT) |
Assignee: |
Megadyne Medical Products, Inc.
(Draper, UT)
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Family
ID: |
34591296 |
Appl.
No.: |
11/537,396 |
Filed: |
September 29, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070049916 A1 |
Mar 1, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10719333 |
Nov 21, 2003 |
7169145 |
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Current U.S.
Class: |
606/35;
606/32 |
Current CPC
Class: |
A61B
18/16 (20130101); A61B 2090/065 (20160201); A61B
18/1233 (20130101) |
Current International
Class: |
A61B
18/04 (20060101) |
Field of
Search: |
;606/32,35,39,152
;128/908 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 480 736 |
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Jul 1977 |
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GB |
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2 052 269 |
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Jan 1981 |
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GB |
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S55-168317 |
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May 1979 |
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JP |
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S57-154409 |
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Sep 1982 |
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JP |
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S57-188250 |
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Nov 1982 |
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JP |
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S63-54148 |
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Mar 1988 |
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JP |
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Other References
Wald, et al., "Accidental Burns Associated With Electrocautery,"
JAMA, Aug. 16, 1978, vol. 217, No. 7, pp. 916-921. cited by other
.
Notice of Allowance dated Jul. 13, 2006, 4 pages, U.S. Appl. No.
10/719,333. cited by other .
Non-Final Office Action dated Aug. 24, 2005,4 pages, U.S. Appl. No.
10/719,333. cited by other .
Notice of Allowance dated Dec. 12, 2005, 6 pages, U.S. Appl. No.
10/719,333. cited by other.
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Primary Examiner: Gibson; Roy D
Attorney, Agent or Firm: Workman Nydegger
Parent Case Text
RELATED APPLICATIONS
This application is a continuation application of co-pending U.S.
patent application Ser. No. 10/719,333, filed 21 Nov. 2003,
entitled "Tuned Return Electrode with Matching Inductor", the
disclosure of which is incorporated herein by this reference.
Claims
What is claimed is:
1. A self-limiting electrosurgical return electrode comprising: a
semi-insulating member for limiting the density of electrosurgical
current flowing through the electrode to less than a predetermined
level to prevent a patient burn; and a conductor member coupled to
the semi-insulating member, the conductor member having a split
plate configuration and coupled to circuitry for monitoring the
contact area between the electrode and a patient, wherein the
circuitry is configured to measure at least one of an
electrosurgical current or electrosurgical current density flowing
through the electrode.
2. The self-limiting electrosurgical return electrode of claim 1,
wherein the conductor member comprises a first conductor
electrically isolated from a second conductor.
3. The self-limiting electrosurgical return electrode of claim 1,
wherein the first conductor and a second conductor interwoven in a
lattice structure
4. The self-limiting electrosurgical return electrode of claim 3,
wherein the circuitry measures the impedance between the first
conductor and the second conductor.
5. The self-limiting electrosurgical return electrode of claim 1,
wherein the conductor member comprises a plurality of membrane
switches.
6. The self-limiting electrosurgical return electrode of claim 1,
wherein the electrode further comprises one or a combination of a
resistive component, a capacitive component, and an inductive
component.
7. A self-limiting electrosurgical return electrode comprising: a
semi-insulating member having a working surface adapted for
disposition adjacent the tissue of a patient positioned thereon for
electrosurgery and that continuously and automatically regulates
the electrosurgical current flowing through the electrode as a
function of the area of contact between the electrode and the
patient's tissue so as to limit the density of electrosurgical
current to less than 100 milliamperes per square centimeter of the
electrode; and a conductor member coupled to the semi-insulating
member, having a split plate configuration, and coupled to
circuitry in electrical communication with the electrically
conducting member, the conductor member and the circuitry
configured to determine the contact area between the electrode and
the patient.
8. The self-limiting electrosurgical return electrode of claim 7,
wherein the semi-insulating member comprises a silicone rubber
material impregnated with conductive fibers.
9. The self-limiting electrosurgical return electrode of claim 7,
further comprising an inductor in electrical series with the
semi-insulating member.
10. The self-limiting electrosurgical return electrode of claim 7,
further comprising a variable inductor in electrical series with
the semi-insulating member to minimize capacitive reactance.
11. The self-limiting electrosurgical return electrode of claim 7,
wherein the first conductor and a second conductor arranged in one
of alternating geometric shapes to aid with determining the amount
of contact area between the patient and the electrode.
12. The self-limiting electrosurgical return electrode of claim 11,
wherein the geometric shape comprises strips, triangles, or
ellipses.
13. A self-limiting electrosurgical system comprising: a
self-limiting electrosurgical return electrode comprising: a
semi-insulating member having a bulk impedance sufficient to
prevent a patient burn when a contact area between a patient and
the semi-insulating member is below a given threshold; and a
conductor member coupled to the semi-insulating member, the
conductor member having a split plate configuration, wherein an
effective impedance of the self-limiting electrosurgical return
electrode is below 100 ohms; and contact quality monitoring
circuitry coupleable to the conductor member, the combination of
the contact quality monitoring circuitry and the conductor member
monitoring the contact area between the semi-insulating member and
the patient and activating an output device if the contact area is
below a predetermined threshold.
14. The self-limiting electrosurgical system of claim 13, further
comprising an electrosurgical generator in electrical communication
with the semi-insulating member.
15. The self-limiting electrosurgical system of claim 13, further
comprising an inductor in series with the electrosurgical return
electrode, the inductor counteracting at least a portion of an
effective impedance of the electrosurgical return electrode and a
patient.
16. The self-limiting electrosurgical system of claim 15, wherein
the inductor is selected such that the effective impedance of the
electrosurgical return electrode, the patient, and the inductor
falls within a range of impedances at which effective
electrosurgery can be performed for a selected group of
patients.
17. The self-limiting electrosurgical system of claim 15, wherein
said inductor is selected from the group consisting of a solid
state inductor, an electro-mechanical inductor, a fixed inductor, a
variable inductor, solid state wave shaping circuitry or any
combination thereof.
18. The self-limiting electrosurgical system of claim 13, wherein
by minimizing the effective impedance to below 100 ohms
electrosurgical surgery on patients weighing less that 25 pounds
can be performed.
19. The self-limiting electrosurgical system of claim 13, wherein
by minimizing the effective impedance to below 100 ohms the
electrosurgical return electrode can be utilized for neonatal
applications.
20. The self-limiting electrosurgical system of claim 13, wherein
the self-limiting electrosurgical return electrode comprises
electrically conducting material having an effective bulk impedance
equal to or greater than about 4,000 .OMEGA.cm.
21. The electrosurgical apparatus of claim 13, wherein the
self-limiting electrosurgical return electrode having an effective
bulk impedance equal to or greater than about 10,000 .OMEGA.cm.
Description
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to electrosurgical systems.
More specifically, the present invention relates to electrosurgical
electrodes that are adapted for providing safe and effective
electrosurgery.
2. The Relevant Technology
As is known to those skilled in the art, modern surgical techniques
typically employ radio frequency (RF) power to cut tissue and
coagulate bleeding encountered in performing surgical procedures.
For historical perspective and details of such techniques,
reference is made to U.S. Pat. No. 4,936,842, issued to D'Amelio et
al., and entitled "Electroprobe Apparatus," the disclosure of which
is incorporated by this reference.
As is known to those skilled in the medical arts, electrosurgery is
widely used and offers many advantages including the use of a
single surgical tool for both cutting and coagulating the tissue of
a patient. Every monopolar electrosurgical generator system must
have an active electrode that is applied by the surgeon to the
patient at the surgical site and a return path from the patient
back to an electrosurgical generator that provides the RF power
used during electrosurgical procedures. The active electrode at the
point of contact with the patient must be small to produce a high
current density resulting in a surgical effect of cutting or
coagulating tissue. The return electrode, which carries the same
current as the active electrode, must be large enough in effective
surface area at the point of communication with the patient so that
the density of the electrosurgical current flowing from the patient
to the return electrode is limited to safe levels. If the density
of the electrosurgical current is relatively high at the return
electrode, the temperature of the patient's skin and tissue will
rise in this area and can result in an undesirable patient
burn.
In 1985, the Emergency Care Research Institute, a well-known
medical testing agency, published the results of testing it had
conducted on electrosurgical return electrode site burns, stating
that the heating of body tissue to the threshold of necrosis occurs
when the current density exceeds 100 milliamperes per square
centimeter. The Association for the Advancement of Medical
Instrumentation ("AAMI") has published standards that require that
the maximum patient surface tissue temperature adjacent an
electrosurgical return electrode should not rise more than six
degrees (6E) Celsius under stated test conditions.
Over the past twenty years, products have been developed in
response to the medical need for a safer return electrode. One
advancement in return electrode technology was the development of a
flexible electrode to replace the small, about 12.times.7 inches,
flat stainless steel plate electrode typically in use during
electrosurgical procedures. This plate electrode was typically
coated with a conductive gel, placed under the patient's buttocks,
thigh, shoulders, or any other location, and relied upon gravity to
ensure adequate contact area. These flexible electrodes, which are
generally about the same size as the stainless steel plates, are
coated with a conductive or dielectric polymer and have an adhesive
border on them so they will remain attached to the patient without
the aid of gravity. By the early 1980's, most hospitals in the
United States were using flexible electrodes. Flexible electrodes
resulted in fewer patient return electrode burns but resulted in
additional surgical costs in the United States of several tens of
millions of dollars each year because each electrode had to be
disposed of after use. Even with this improvement, hospitals were
still experiencing some patient burns caused by electrodes that
would accidentally fall off or partially separate from the patient
during surgery.
In an attempt to minimize the potential for patient burns, contact
quality monitoring systems were developed. Contact quality
monitoring systems are adapted to monitor the contact area of an
electrode that is in contact with a patient and turn off the
electrosurgical generator whenever there is insufficient contact
area between the patient and the electrode. Such circuits are
shown, for example, in U.S. Pat. No. 4,200,104 issued to Harris,
and entitled "Contact Area Measurement Apparatus for Use in
Electrosurgery" and; U.S. Pat. No. 4,231,372, issued to Newton, and
entitled "Safety Monitoring Circuit for Electrosurgical Unit," the
disclosures of which are incorporated by this reference. Contact
Quality Monitoring Systems have resulted in additional reduction in
patient return electrode burns, but require special disposable
electrodes, resulting in an increase in the cost per procedure.
Twenty years after these systems were first introduced, only 75
percent of all the surgical operations performed in the United
States use contact quality monitoring systems because of the
increased costs and other factors.
Self-limiting electrosurgical return electrodes provide an
alternative to contact quality monitoring systems. Self-limiting
electrosurgical return electrodes allow electrosurgery to be
performed when the contact area between the patient and the pad is
sufficient to limit the current density of the electrosurgical
current to safe levels and when there are not too many materials
placed between the patient and the pad. When the contact area
between the patient and the return electrode falls below a minimum
contact area or when too many materials are placed between the
patient and the pad, the properties of the pad limit the flow of
current to prevent a patient burn.
While self-limiting electrodes are typically reusable and provide
current limiting when the contact area between the patient and the
electrode falls below a minimum contact area or too many materials
are placed between the patient and the pad, the impedance
properties of the pad can result in current limiting of the
electrosurgical current under some conditions. For example, during
surgeries that require high current flow such as trans-urethral
resection of the prostate procedures (TURP), though the contact
area may be sufficient to conduct safe electrosurgery, small
increases in impedance can noticeably affect the current flow.
Additionally, procedures involving small pediatric patients can
result in diminished current flow due to the relatively small
contact area of the patient with the pad and the resulting
increases in impedance. This is particularly true for neonatal
patients, where the small size and mass of the patients have
rendered present applications impractical.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to an electrosurgical return
electrode adapted to prevent patient burns. The return electrode
provides a bulk impedance that provides self-limiting properties to
the electrode. The bulk impedance of the electrosurgical return
electrode allows the return electrode to be self-limiting and can
result from the properties of the semi-insulating member, the
conductor member, a combination of both the semi-insulating member
and the conductor member, or a combination of two or more of the
semi-insulating member, the conductor, clothing of the patient,
blankets, sheets, and other materials that are disposed between the
patent and the return electrode.
According to one aspect of the present invention, an inductor is
coupled in series with a capacitive electrosurgical return
electrode. The inductor is configured to optimize the flow of the
electrosurgical current by minimizing the effective impedance of
the electrosurgical return electrode when the amount of contact
area between the patient and the electrosurgical return electrode
is sufficient to conduct electrosurgery or where materials are
placed between the patient and the electrosurgical return
electrode.
According to another aspect of the present invention, a capacitor
is coupled in series with an inductive electrosurgical return
electrode. The capacitor is configured to optimize the flow of the
electrosurgical current by minimizing the effective impedance of
the electrosurgical return electrode when the amount of contact
area between the patient and the electrosurgical return electrode
is sufficient to conduct electrosurgery or where materials are
placed between the patient and the electrosurgical return
electrode.
According to another aspect of the present invention, the
electrosurgical return electrode has a bulk impedance sufficient to
prevent a patient burn when the contact area between the patient
and the electrode is below a given threshold. The conductor member
is adapted for use with circuitry that indicates to a user when the
contact area between the patient and the self-limiting member
and/or return electrode is below a given threshold.
These and other objects and features of the present invention will
become more fully apparent from the following description and
appended claims, or may be learned by the practice of the invention
as set forth hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
To further clarify the above and other advantages and features of
the present invention, a more particular description of the
invention will be rendered by reference to specific embodiments
thereof that are illustrated in the appended drawings. It is
appreciated that these drawings depict only typical embodiments of
the invention and are therefore not to be considered limiting of
its scope. The invention will be described and explained with
additional specificity and detail through the use of the
accompanying drawings in which:
FIG. 1 is a perspective view of an electrosurgical system
illustrating an inductor connected in series with an
electrosurgical return electrode;
FIG. 2 is a top view of a return electrode illustrating the
principles by which impedance varies as a function of contact
area.
FIG. 3 is a schematic view illustrating the impedances presented to
an electrosurgical current and inductor coupled in series
therewith.
FIG. 4 is a chart illustrating in graphical form the relationship
between capacitive reactance, inductive reactance, and frequency of
an electrosurgical current.
FIG. 5A is a perspective view illustrating a representative patient
in contact with a semi-insulating member of a return electrode.
FIG. 5B is a chart illustrating in graphical form the relationship
between the effective impedance of a return electrode, the contact
area between a patient and a return electrode, and the effect of an
inductor on the effective impedance.
FIG. 6 is a block diagram illustrating an electrosurgical power
unit having a tunable variable inductor.
FIG. 7 is a flow diagram illustrating a method for utilizing a
variable inductor to change the amount of inductance based on the
amount of contact area between the patient and the electrosurgical
return electrode.
FIG. 8 is a perspective view of an electrosurgical return electrode
for use with a contact quality monitoring apparatus having a
semi-insulating member and conductor members according to the
present invention.
FIG. 9 illustrates a conductor member having a first and second
conductor arranged in matrix of alternating segments.
FIG. 10 illustrates a conductor member having a first conductor and
a second conductor interwoven in a lattice structure.
FIG. 11A,B illustrate a first conductor and a second conductor that
are configured to comprise a conductor member.
FIG. 12 is a perspective view illustrating a conductor member
having a plurality of membrane switches.
FIG. 13 is a cross-sectional exploded view illustrating the
components of a membrane switch that can be utilized in connection
with the conductor member of FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An electrosurgical return electrode is provided having a bulk
impedance sufficient to prevent a patient burn when the contact
area between the patient and the electrode is below a given
threshold. According to one aspect of the invention, an inductor is
coupled in series with a capacitor as part of an electrosurgical
circuit. In the embodiment, the electrosurgical return electrode
can comprise a capacitive electrosurgical return electrode that is
utilized with a series inductor. Alternatively, the electrosurgical
return electrode can comprise an inductive electrosurgical return
electrode that is utilized with a series capacitor. Where a series
inductor is utilized, the inductor is configured to optimize the
flow of the electrosurgical current by minimizing the effective
bulk impedance of the electrosurgical return electrode when the
amount of contact area between the patient and the electrosurgical
return electrode is sufficient to conduct electrosurgery. According
to another aspect of the present invention, a conductor member is
adapted for use with circuitry that indicates to a user when the
contact area between the patient and the return electrode is below
a given threshold.
Series Inductor
With reference now to FIGS. 1-7, consider a capacitive
electrosurgical return electrode utilized with a series inductor to
minimize the effective impedance of the electrosurgical return
electrode. While a complete discussion of the series capacitor for
use with an inductive electrosurgical return electrode is not
included, as will be appreciated by those skilled in the art, the
principles discussed with reference to the series inductor employed
with a capacitive electrosurgical return electrode can be utilized
to minimize the bulk impedance of an inductive electrosurgical
return electrode with a series capacitor.
With reference now to FIG. 1, there is shown an electrosurgical
system 1 having an inductor coupled in series with a return
electrode to minimize the effective bulk impedance of the return
electrode. As depicted, system 1 includes a return electrode 10, an
electrosurgical generator 12, and an inductor 20. There is also
shown a member 14, a member 16, and an electrosurgical tool 18.
Electrosurgical generator 12 generates an electrosurgical current
(i.e. radio frequency (RF) energy) that is conveyed to
electrosurgical tool 18 by means of member 14. Electrosurgical tool
18 is configured to utilize the electrosurgical current during a
procedure to cut and coagulate tissue of a patient resting on the
return electrode 10. Various types of electrosurgical generator 12
are known to those skilled in the art in light of the teaching
contained herein. The electrosurgical current is returned to
electrosurgical generator 12 utilizing member 16 as the return
path. In the illustrated embodiment, members 14 and 16 comprise
cabling that operate as conductors of the electrosurgical
current.
Return electrode 10 is adapted to limit the density of
electrosurgical current flowing from a patient resting on the
return electrode 10 back to the electrosurgical generator. Return
electrode 10 is adapted to provide self-limiting properties to
prevent patient burns. The self-limiting properties of return
electrode 10 effectively increase the effective impedance of return
electrode 10 to reduce current from flowing when there is
insufficient contact area between the patient and return electrode
10. By reducing current from flowing, use of electrosurgical tool
18 is inhibited and the possibility of patient burns is minimized.
Illustrative materials and geometries for return electrode 10 are
described in U.S. Pat. No. 6,454,764 entitled "Self-Limiting
Electrosurgical Return Electrode" and other related patent
applications, the disclosures of which is incorporated herein by
reference.
Inductor 20 is connected in series with electrosurgical electrode
10. Inductor 20 is configured to minimize the effective impedance
of the electrosurgical current when the amount of contact area
between the patient and the electrosurgical return electrode is
sufficient to conduct electrosurgery, thus optimizing the flow of
the electrosurgical current. As will be appreciated by those
skilled in the art, the relationship between the power delivered by
an electrosurgical current and the impedance of an electrosurgical
current is depicted by a power curve. The power curve indicates
that as the impedance of the electrosurgical circuit increases the
power delivered by the electrosurgical current decreases. The
specific relationship between the power delivered and the impedance
is dependent on the properties of the components of the
electrosurgical circuit. Because the impedance at the surgical
interface is an important source of impedance in an electrosurgical
circuit, the effective impedance of the electrosurgical return
electrode largely determines the amount of power delivered by the
electrosurgical current. The impedance of the electrosurgical
return electrode can be the product of one or a combination of a
resistive component, a capacitive component, and an inductive
component. Inductor 20 is capable of reducing the overall impedance
of the electrosurgical circuit by counteracting the capacitive
component of the effective impedance of the electrosurgical return
electrode. Reduced overall impedance of the electrosurgical circuit
results in increased flow of electrosurgical current and a
resultant increase in power delivered by the electrosurgical
current.
A variety of different types and configurations of inductors can be
utilized in light of the present invention including, but not
limited to, a solid state inductor, or an electromechanical
inductor. In the illustrated embodiment inductor 20 is coupled to
member 14. As will be appreciated by those skilled in the art,
inductor 20 can be placed in a variety of positions within the
system and in a variety of configurations without departing from
the scope and spirit of the present invention. For example, the
conductor can be placed in member 16, electrosurgical tool 18, or
electrosurgical generator 12.
Now turning to FIG. 2, there will be seen a schematic
representation of the top view of a return electrode 10
illustrating the self-limiting principles of return electrode 10.
The effective impedance of return electrode 10 and its relationship
to self-limiting principles illustrates the manner in which an
inductor can be utilized to minimize the effective impedance of the
electrosurgical return electrode. For instructional purposes of
this description and to aid in the mathematical modeling of
electrode 10, electrode 10 may be thought of as including a
plurality of uniformly sized, continuous regions or segments as
represented by regions 11a, 11b, 11c . . . 11n. One skilled in the
art will appreciate, however, that electrode 10 may include
discontinuous regions or segments.
It is known that, in contrast with the series circuit, combined
resistive and capacitive reactances, when connected in parallel,
present a total effective impedance that is given by the
formula:
##EQU00001##
Thus, if 100 similar impedances, each of 100 ohms, were connected
in parallel, the effective impedance Z.sub.eff would equal one ohm.
If half of such impedances were effectively disconnected, the
remaining effective impedance would be two ohms, and if only one of
the impedances were active in the circuit, the remaining effective
impedance would be 100 ohms. As a result, the total effective
impedance of electrode 10 is rendered self-limiting due to
properties of capacitors, resistors, and inductors in parallel.
Each of the segments of electrode 10 corresponding to segments 11a
. . . 11n inherently has the capability of presenting an impedance.
However, the number of such segments that are effectively active in
parallel within the circuit is a direct function of the surface
area of the patient that overlies the electrode. Thus, in the case
of a large supine patient whose body is in effective contact with
50 percent (50%) of the upper surface of electrode, 50 percent of
the segments corresponding to segments 11a-11n will be effectively
in parallel in the circuit to form a given impedance. Where
electrode 10 contains 100 segments of 1000 ohms each, the effective
impedance operatively presented by the effective 50 percent of the
electrode elements would be 20 ohms. Since 20 ohms is very small
compared with the impedance at the surgical interface, very little
energy is dissipated at the region of contact between the patient
and electrode 10, and due also to the relatively large effective
working area of electrode 10, current density, and temperature
elevation are maintained below the danger thresholds mentioned
above.
Now, if for any reason, the effective contact area between the
patient and electrode 10 were to be reduced to the surface of only
one of segments 11a-11n, then the effective impedance would
increase to 1000 ohms. At some point of reduction in contact area,
the effective impedance would rise to a level relative to the
impedance presented at the site of electrosurgical tool or
instrument 18 (FIG. 1) to diminish the electrosurgical effect of
tool or instrument 18 or otherwise prevent effective use of tool or
instrument 18 by the surgeon. This diminishing of electrosurgical
effect or effectiveness of tool or instrument 18 signals to the
surgeon that the patient should be repositioned so as to present a
greater surface area in contact with return electrode 10. As the
effective impedance rises, the total circuit impedance would be
increased so that the total current that would flow if the surgeon
attempted to employ tool or instrument 18 without repositioning the
patient would be reduced to a value below that which would cause
undesired trauma to the patient.
When the effective contact area is large, the current at the
surgeon's implement is high and the corresponding current density
across return electrode 10 low. This is the condition desired for
performing surgery. However, as the effective surface area
decreases, the impedance of return electrode 10 increases with a
corresponding decrease in the current at tool or instrument 18
(FIG. 1). When the effective surface area declines to some
predetermined point, there will remain insufficient current at tool
or instrument 18 to effectively conduct surgery. The parameters
selected for the materials and dimensions of electrode 10 are
chosen so that current density and corresponding tissue temperature
elevation adjacent return electrode 10 does not exceed the limits
mentioned in the introduction hereof. For example, in one
embodiment return electrode 10 has a bulk impedance of at least
4,000 .OMEGA.cm so as to limit the current density to safe levels.
To facilitate description of the principles underlying the
invention, the foregoing is described in terms of impedances whose
principal components are resistances and capacitive reactances.
However, the principles of the invention are also applicable to
other embodiments in which the impedances include any combination
of resistive, capacitive and/or inductive impedances.
By providing a return electrode 10 having both the desired bulk
impedance and a sufficient surface area, the electrosurgical
current is distributed sufficiently such that the current density
does not result in a patient burn. It has been found that with
selected materials and geometries, the self-limiting principles
hereof can be achieved in a return electrode as small as about
seven square inches (or about 45 square centimeters) in working
surface area, while the preferable range of exposed upper working
surface area of return electrode 10 lies in the range of from about
11 to 1,500 square inches (or about 70 to 9,680 square
centimeters).
Return electrode 10 need not be in direct physical contact with the
patient. Having a working surface area of this size eliminates the
need for direct physical attachment, either directly to the skin of
the patient or through gels. A patient can be in electrical
connection with return electrode 10 without requiring the use of
adhesives or gels. This also allows return electrode 10 to be
re-used thereby eliminating the need and cost of disposable
split-plate electrodes that are commonly used. This reduces the
cost for using contact quality monitoring techniques to verify that
the patient is sufficiently in contact with a return electrode to
prevent high current densities that result in patient burns.
Additionally, it can be understood that the self-limiting
characteristics or capabilities of return electrode 10 can be
achieved where return electrode 10 is substantially enclosed within
a semi-insulating member. Additionally, the self-limiting
characteristics or capabilities can be provided in part, from
materials, members or elements disposed between return electrode 10
and a patient. For instance, such other materials, members, or
elements can include but are not limited to, sheets, clothing,
blankets, or the like. Therefore, electrode 10 has an effective
bulk impedance sufficient to prevent a patient burn when the
contact area between the patient and electrode 10 is below a given
threshold.
The electrode 10 according to the invention hereof may be made of
conductive plastic, rubber, or other flexible material which, when
employed in electrode 10 will result in an effective DC resistance
presented by each square centimeter of working surface sufficient
to limit the current density to safe levels. Silicone or butyl
rubber have been found to be particularly attractive materials as
they are flexible, as well as readily washable and sterilizable.
Alternatively, a portion of return electrode 10 may be made of
inherently relatively high resistance flexible material altered to
provide the requisite conductivity. For example, a silicone rubber
material in which there are impregnated conductive fibers, such as
carbon fiber, or in which there have been distributed quantities of
other conductive substances such as carbon black, quantities of
gold, silver, nickel, copper, steel, iron, stainless steel, brass,
aluminum, or other conductors. A more complete discussion of
self-limiting characteristics can be found in U.S. Pat. No.
6,454,764 entitled "Self-Limiting Electrosurgical Return
Electrode," which is incorporated herein by reference.
With reference now to FIG. 3, there is shown a simplified
electrical schematic diagram of an electrosurgical circuit
illustrating the manner in which an inductor can be utilized to
minimize the effective impedance of a return electrode. There are
shown the typical impedances z.sub.1, z.sub.2, and z.sub.3
effectively included in the operative path of an electrosurgical
current during an operative procedure and an inductor 20 connected
in series therewith. The inductor 20 is configured to minimize the
effective impedance of the electrosurgical return electrode when
the amount of contact area between the patient and the
electrosurgical return electrode is safe with regard to current
densities.
Electrosurgical generator 12 is adapted to provide an
electrosurgical current, such as but not limited to constant flow,
voltage, and/or current or variable flow, voltage and/or current.
Connected to electrosurgical generator 12 are conventional
electrical conductors 14 and 16 which respectively connect the
generator 12 to the electrosurgical tool 18 represented by
impedance z.sub.1 (at the surgical interface) and an return
electrode 10 represented by impedance z.sub.3. Impedance z.sub.2 is
provided to represent the impedance presented by the patient's
tissue lying between the operation site and the return
electrode.
The diagram of FIG. 3 is a simplified version of the
electrosurgical current circuit. The diagram generally considers
circuit elements in terms of the principal impedances, including
the impedances contributed by the surgical interface, the patient's
body, and the return electrode, so as to clearly and succinctly
illustrate principles of the invention, it should be understood
that in reality certain other parameters would be encountered,
parameters such as distributed inductance and distributed
capacitance which, for purposes of clarity in illustration of the
principles hereof, are deemed relatively small and so are not
considered in this description.
The initial embodiment, hereof, is that of an electrode operating
in an exclusive capacitive mode or a combined resistive and
capacitive mode. Accordingly, if the relatively small stray
capacitive and inductive reactances are disregarded, the total
effective impedance of the circuit will be equal to the sum of the
individual impedances z.sub.1, z.sub.2 and z.sub.3 minus the series
inductor; and since essentially the same current will pass through
all three, the voltage generated by electrosurgical generator 12
will be distributed across impedances z.sub.1, z.sub.2 and z.sub.3
in direct proportion to their respective values. Thus, the energy
dissipated in each of such components will also be directly
proportional to their values.
Since it is desired that developed energy be concentrated in the
region where the surgeon's implement contacts the patient's tissue,
it is desirable that the resistive component of the impedance
represented by z.sub.1 be substantial and that current passing
therethrough (and consequent energy dissipation) be concentrated in
a very small region. The latter is accomplished by making the
region of contact with the patient at the operative site very
small.
In contrast to the region where the surgeon's implement contacts
the patient's tissue, it desired that the effective impedance
z.sub.3 of the return electrode be minimized and that the current
passing therethrough be distributed in a large region to avoid an
undesirable patient burn. Accordingly, it is desired that the
contact area between the patient and the return electrode 10 be
substantial (compared to the region where the surgeon's implement
contacts the patient's tissue) and the effective impedance of the
return electrode be small. Return electrode 10 is rendered
self-limiting to ensure that the current density of the current
passing therethrough is limited so as not to result in a patient
burn. As will be appreciated by those skilled in the art, a variety
of combinations of resistive components, capacitive components,
and/or inductive components can be utilized to achieve the
self-limiting characteristics or capabilities of return electrode
10.
As previously discussed, inductor 20 is coupled in series with
return electrode 10 and impedance z.sub.3 presented thereby.
Inductor 20 is configured to counteract the capacitive component of
the effective impedance z.sub.3 of the electrosurgical return
electrode. The impedance of the return electrode 10 can be
presented by a resistive component, a capacitive component, and/or
an inductive component, as shown by the following equations:
.omega..times..times. ##EQU00002## where X.sub.c is the capacitive
reactance, j is the vector direction of the capacitive reactance
and is equal to 1/ -1, .omega. is the frequency in Hertz of the
electrosurgical current multiplied by 2.pi., C is the capacitance
in Farads; X.sub.L=j.omega.L (2) Where X.sub.L is the inductive
reactance, j is the vector direction of the inductive reactance and
is equal to 1/ -1, .omega. is the frequency in Hertz of the
electrosurgical current multiplied by 2.pi., and L is the
inductance in millihenrys (mH). The total impedance of return
electrode 10 is the sum of the resistive component, the capacitive
component, and the inductive component and is given by the
formula:
.omega..times..times..omega..times..times. ##EQU00003##
By changing the phase angle (represented by the symbol j), it is
possible to utilize the inductive reactance to reduce the amount of
capacitive reactance of the electrosurgical electrode, as shown in
the following equation:
.times..omega..times..times..omega..times..times. ##EQU00004##
Equation 5 illustrates that, by changing the phase of the inductive
reactance, the phase angle can be factored out of the inductive and
capacitive reactances. Once the phase angle is factored out, the
inductive reactance can be subtracted from the capacitive
reactance. Thus, by changing the phase angle expression of the
inductive reactance, the inductive reactance can be utilized to
counteract the effective capacitive reactance presented by the
return electrode 10. In other words, the phase angle of the
inductance can be utilized to minimize the capacitive reactance of
the parallel plate capacitor when the amount of contact area
between the patient and the electrosurgical return electrode is
sufficient to limit the density of the electrosurgical current to
safe levels.
With reference now to FIG. 4, there is shown the relationship
between frequency of an electrosurgical current flowing through the
return electrode and the reactance of a capacitor and an inductor.
However, before proceeding to a consideration of such chart, it
should be noted that the chart is simplified so as to illustrate
the principles underlying the invention and does not represent
actual data that may vary substantially. The graph illustrates that
the magnitude of the inductive reactance and capacitive reactance
vary according the frequency of the electrosurgical current. The
inductive reactance varies in proportion to the frequency of the
electrosurgical current, while the capacitive reactance varies in
inverse proportion to the frequency of the electrosurgical current.
This is due to the fact that inductive reactance and capacitive
reactance are determined using .omega. represented by the equation:
.omega.=2.pi.f (5) where .pi. is 3.14159, f is frequency in
hertz.
Where the frequency of the electrosurgical current is constant, the
amount of inductive reactance can be established by simply
selecting an inductor 20 having a desired amount of inductance. Due
to the fact that electrosurgical generators typically provide an
electrosurgical current having a consistent frequency, the
frequency is an ascertainable constant. Where the capacitance of
the parallel plate capacitor and the frequency are also known, a
selected inductive reactance can be utilized to minimize the
orthogonal reactance of the return electrode 10 relative to
resistance. However, the capacitive reactance can be difficult to
establish due to the fact that the self-limiting electrode is
typically utilized such that the amount of contact area between the
patient and the return electrode is variable. Additionally, the
capacitive reactance can be affected by materials positioned
between the patient and the electrosurgical return electrode. The
relationship between contact area, interposed materials, and
capacitive reactance is discussed in greater detail with reference
to FIGS. 5A and 5B.
In selecting a desired amount of inductive reactance, a user can
determine an ideal capacitive reactance X.sub.cIdeal based on the
desired contact area and properties of the materials between the
patient and the electrosurgical return electrode. The relationship
of capacitive reactance for X.sub.cIdeal relative to frequency is
depicted in FIG. 4. Once the amount of capacitive reactance for
X.sub.cIdeal is determined for the frequency of the electrosurgical
generator, an inductor can be selected that provides a desired
amount of inductive reactance to counteract the capacitive
reactance of X.sub.cIdeal. The point of intersection of
X.sub.cIdeal and X.sub.L indicates the frequency where the
reactances of X.sub.cIdeal and X.sub.L counteract one another. As a
result, where the actual capacitive reactance of the
electrosurgical circuit is X.sub.cIdeal, the series inductor will
counteract the capacitive reactance and the overall impedance will
be reduced by the magnitude of the capacitive reactance.
However, where the contact area and/or the materials between the
patient and the electrosurgical return electrode vary from the
desired contact area and/or the desired properties of the materials
between the patient and the electrosurgical return electrode, the
capacitive reactance will vary from X.sub.cIdeal as is shown with
respect to X.sub.cnon-ideal. Where the capacitive reactance is
represented by X.sub.cnon-ideal rather than by X.sub.cIdeal, the
inductive reactance will continue to counteract the capacitive
reactance presented by the electrosurgical circuit. However, the
reduction in the overall impedance will not be reduced by the
magnitude of the actual capacitive reactance of the circuit.
Instead, the overall impedance of the electrosurgical circuit will
be reduced by an inductive reactance provided by the inductor that
is different from the magnitude of the actual capacitive reactance
of the electrosurgical circuit. Where the capactive reactance is
greater than inductive reactance, as with X.sub.cnon-ideal, a
reduced net positive capacitive reactance will be produced. Where
the capacitive reactance is less than the inductive reactance, a
net inductive reactance will be produced.
As will be appreciated by those skilled in the art, a series
capacitor can be utilized with a self-limiting electrosurgical
return electrode having an inductive component without departing
from the scope and spirit of the present invention. A series
capacitor can be utilized relying on the principles describe with
reference to FIG. 4. In the embodiment, the series capacitor
provides a level of capacitive reactance needed to counteract the
inductive reactance of the electrosurgical return electrode. A
variety of type and configuration of the series capacitors can be
utilized without departing from the scope and spirit of the present
invention.
With reference now to FIG. 5A, there is shown a schematic
representation of return electrode 10 and a patient in contact
therewith. FIG. 5A is utilized to illustrate the relationship
between the contact area and the capacitive reactance in order to
describe how an inductor can be utilized to minimize the capacitive
reactance of the return electrode while maintaining the
self-limiting properties of the return electrode 10. There is shown
a conducting layer 60 and a return electrode 10. In the illustrated
embodiment, return electrode 10 comprises a semi-insulating member
30 and an electrically conductive member 32. Conducting layer 60
represents a patient resting on a semi-insulating member 30.
Conducting layer 60 is configured to represent the minimum contact
area required to limit the current density to safe levels.
As discussed with reference to FIG. 2, the number of such segments
that are effectively active in parallel within the circuit is a
direct function of the surface area of the patient that overlies
return electrode 10. Where the surface area of the patient that
overlies electrode 10 is at, or above, the minimum contact area,
the total effective impedance is sufficiently low to permit the
electrosurgical current to conduct safe and effective
electrosurgery. Where the impedance is due primarily to a
capacitive component and a resistive component, the amount of
impedance is inversely proportional to the amount of patient
contact area.
While the effective impedance is sufficiently low to conduct safe
electrosurgery, under some conditions the effective impedance
resulting from the contact area and the properties of the pad can
result in current limiting of the electrosurgical current. This is
often the result of a bulk impedance of the pad that exceeds 10,000
ohms centimeter. For example, during surgeries that require high
current flow such as trans-urethral resection of the prostate
procedures (TURP), small increases in impedance can noticeably
affect the current flow. Additionally, procedures involving small
pediatric patients can result in diminished current flow due to the
contact area of the patient with the pad and the resulting
increases in impedance. This is particularly true for neonatal
patients, where the small size and mass of the patients have
rendered present applications impractical.
By placing inductor 20 (see FIG. 1) in series with return electrode
10, the effective impedance of the return electrode can be
minimized. For example, during surgeries that require high current
flow, inductor 20 can counteract the capacitive reactance component
of the effective impedance of the return electrode. By
counteracting the capacitive reactance, only the resistive
component of the bulk impedance remains (assuming little or no
inductive reactance in the return electrode.) The capacitive
reactance is a function of several factors including the contact
area of the patient to the return electrode. Where the patient
contact area is greater than, or equal to, the minimum contact
area, or where the materials between the patient and the
electrosurgical return electrode are minimal, the effective
impedance of the return electrode is often in the range of 100
ohms. The inductor is configured to reduce the effective impedance
of the electrosurgical electrode below 100 ohms. Where the majority
of the effective impedance of the return electrode is due to
capacitive reactance, an inductor providing a desired amount of
inductance can be utilized to eliminate the majority of the
effective impedance of the return electrode. By minimizing the
effective impedance of the pad, surgeries that are sensitive to
small changes in the effective impedance of the return electrode,
such as pediatric, neonatal, and TURP procedures can be performed
with minimal reduction in the current flow.
The capacitive reactance of the return electrode is determined in
order to identify the amount of inductance to be provided by the
inductor. As previously discussed, the capacitive reactance of a
split electrode is defined by the equation:
.omega..times..times. ##EQU00005## While the frequency of a
self-limiting return electrode can be controlled without
difficulty, the amount of capacitance C can be more complicated to
control.
The capacitance for a parallel plate capacitor is defined as:
.times..times..times. ##EQU00006## where C is capacitance in
Farads, .kappa. is the dielectric constant of the material lying
between the effective plates of the capacitor, A is the area of the
smallest one of the effective plates of the capacitor in square
meters, t is the separation of the surfaces of the effective plates
in meters, and .epsilon..sub.0 is the permittivity of air in
Farads/meter. There are two primary mechanisms by which the
capacitance C can be varied: 1) patient contact area A (i.e. the
area of the smallest one of the effective plates of the capacitor
in square centimeters); and 2) materials lying between the patient
and the return electrode (i.e. which can affect both .kappa. the
dielectric constant of the material lying between the effective
plates of the capacitor and t the separation of the surfaces of the
effective plates in meters.) By providing parameters to control the
variability in materials positioned between the patient and the
return electrode 10, .kappa. the dielectric constant of the
material lying between the effective plates of the capacitor,
.epsilon..sub.0 the permittivity of air in Farads/meter, and t the
separation of the surfaces of the effective plates in meters will
all be constants. However, due to the manner in which return
electrode 10 will typically be utilized, the patient contact area A
(i.e. the area of the smallest one of the effective plates of the
capacitor in square centimeters) will be variable. As will be
appreciated by those skilled in the art, the area of the smallest
one of the effective plates of the capacitor is the equivalent of
the contact area between the patient and the return electrode.
Due to the self-limiting aspects of return electrode 10 and the
manner in which the self-limiting return electrode 10 is utilized,
the contact area between the patient and the return electrode will
vary during the course of a medical procedure. The bulk impedance
of the pad allows a user to counteract the capacitive component of
the effective bulk impedance of the electrosurgical return
electrode 10 when the amount of contact area is safe with respect
to current densities while maintaining the self-limiting aspect of
the return electrode when the contact area is reduced. The ability
to counteract the orthogonal impedance of the return electrode
while maintaining the self-limiting aspect of the return electrode
is shown in greater detail with reference to FIG. 5B.
With reference now to FIG. 5B, there is shown a chart illustrating
in graphical form the relationship between the effective impedance
of a return electrode, the contact area between a patient and a
return electrode, and the influence of an inductor on the effective
impedance. However, before proceeding to a consideration of such
chart, it should be noted that the chart is simplified so as to
illustrate the principles underlying the invention and does not
represent actual data that may vary substantially. The line graphs
illustrate the effective impedance of a return electrode as a
function of contact area between the patient and the return
electrode. The upper line graph represents the effective impedance
of a return electrode where no inductor is coupled in series with
the return electrode. The lower line graph represents the effective
impedance of a return electrode where an inductor is coupled in
series with the return electrode.
The effective impedance of the return electrode is inversely
proportional to the contact area A. Where the patient contact area
is less than the minimum contact area (A.sub.contact(min)) the
effective impedance of the return electrode increases sharply.
However, where the patient contact area is greater than the
A.sub.contact(min) there is minimal change in the effective
impedance of the return electrode. The minimal change in the
effective impedance of the return electrode when the contact area
is greater than A.sub.contact(min) allows a inductor having a set
amount of inductance to minimize most of the capacitive reactance
of the return electrode. However, the sharp increase in the
effective impedance of the return electrode when the contact area
is less than A.sub.contact(min) limits the ability of the inductor
to minimize the capacitive reactance of the return electrode. Where
the amount of contact area between the patient and the return
electrode is sufficient to conduct effective electrosurgery, the
inductor coupled in series with the return electrode counteracts
the effective bulk impedance of the electrosurgical return
electrode. Where the amount of contact area between the patient and
the return electrode is insufficient to conduct safe
electrosurgery, the self-limiting aspects of the return electrode
is maintained and the electrosurgical current is limited to safe
levels. In other words, the inductive reactance provided by the
inductor is selected to be insufficient to minimize the capacitive
reactance of the return electrode when the capacitive reactance is
greater than a threshold level. The capacitive reactance is greater
than the threshold level when the contact area between the patient
and the return electrode is not safe with respect to current
densities.
Variable Inductor
With reference now to FIG. 6, there is shown a variable inductor
20a connected in series with a return electrode 10. There is also
shown an electrosurgical power unit 50 having a logic module 58
adapted to tune variable inductor 20a to optimize the flow of the
electrosurgical current by minimizing the capacitive reactance in
the electrosurgical pathway. In the illustrated embodiment,
electrosurgical power unit 50 comprises an electrosurgical
generator 52, a sensor 54, a user input module 56, and a logic
module 58. Variable inductor 20a is positioned internal to
electrosurgical power unit 50. There is also shown an
electrosurgical tool 18 and a return electrode 10 connected in
series with the variable inductor 20a. The apparatus of FIG. 6 is
but one example of a mechanism for controlling the variable
inductor. As will be appreciated by those skilled in the art, a
variety of types and configurations of mechanisms can be utilized
to control the variable inductor without departing from the scope
and spirit of the present invention.
Variable inductor 20a is configured to provide different amounts of
inductance in the electrosurgical pathway. This allows the amount
of inductive reactance to be varied as the capacitive reactance
varies. As discussed with reference to FIGS. 5A and 5B, the
capacitive reactance varies as a function of the contact area and
the materials between the patient and the return electrode 10. Due
to the manner in which self-limiting return electrodes are
typically used, the contact area and the amount of capacitive
reactance in the electrosurgical pathway will often fluctuate. By
utilizing variable inductor 20a, the amount of inductance can be
changed corresponding with changes in the capacitive reactance to
provide optimal levels of electrosurgical current flow. In the
preferred embodiment, the amount of inductance that can be provided
by the variable inductor is limited such that the capacitive
reactance can only be minimized when the contact area between the
patient and the return electrode is greater than the minimum
contact area. This allows the variable inductor to counteract the
capacitive reactance of the return electrode when the patient is in
sufficient contact area with the electrosurgical electrode to
perform safe and effective electrosurgery. However, when the
contact area is less than the minimum contact area, the effective
impedance of the pad is sufficient to limit the electrosurgical
current to safe levels.
As will be appreciated by those skilled in the art, a variety of
types and configurations of variable inductors can be utilized to
provide varying amounts of inductance in the electrosurgical
pathway. For example, in one embodiment, the variable inductor 20
comprises a plurality of inductors that are configured to be
utilized alone, or in combination, to provide varying amounts of
inductance in the electrosurgical pathway, with each inductor
providing a set amount of inductance. In an alternative embodiment,
the variable inductor comprises an electromechanical inductor that
is regulated by a control module to provide varying amounts of
inductance.
Sensor 54 and logic module 58 are adapted to determine the amount
of capacitive reactance in the electrosurgical pathway and tune the
variable inductor to optimize the flow of the electrosurgical
current by minimizing the capacitive reactance. Sensor 54 is
configured to identify the properties of the electrosurgical
current returning to the electrosurgical power unit 50 from return
electrode 10. Sensor 54 then relays the information regarding the
properties of the electrosurgical current to logic module 58. Logic
module 58 utilizes the properties of the electrosurgical current to
determine the amount of impedance in the electrosurgical pathway
and calculate the amount of capacitive reactance in the
electrosurgical pathway. Once the amount of impedance in the
electrosurgical pathway is determined, the logic module tunes the
variable inductor 20 to provide a desired amount of inductive
reactance to minimize the capacitive reactance in the
electrosurgical pathway. A variety of types and configurations of
sensors and logic modules can be utilized within the scope and
spirit of the present invention. For example, in one embodiment,
the sensor and the logic module are integrated in a microprocessor.
In an alternative embodiment, the sensor and logic module comprise
separate hardware circuitry.
User input module 56 is configured to allow a user to provide input
to logic module 58 to control the amount of inductance provided by
variable inductor 20. The functionality, configuration, and purpose
of user input module can be tailored to the needs of the user. For
example, the user input module can include a button allowing the
user to place the electrosurgical power unit in a condition
preferred for specialize procedures such as neonatal surgeries or
TURP procedures. When the electrosurgical power unit is in a
condition preferred for specialize procedures, logic module 58 a
tunes variable inductor 20 to minimize the impedance to the extent
required, or based on special properties of the electrosurgical
apparatus employed, for those procedures.
With reference now to FIG. 7, there is shown a method for utilizing
a variable inductor to provide an amount of impedance based on a
patient contact area. According to the method, electrosurgery is
started in step 80. Once electrosurgery is started, the properties
of the electrosurgical current are identified in step 82. Based on
the properties of the electrosurgical current, the effective
impedance exhibited by the electrosurgical pathway is determined in
step 82. Based on the effective impedance exhibited by the
electrosurgical pathway, the amount of capacitive reactance of the
return electrode is calculated in step 86. Using the amount of
capacitive reactance of the return electrode, the amount of
inductive reactance needed to minimize the impedance of the return
electrode is determined in step 88. The variable inductor is then
tuned to provide the amount of inductance necessary to realize the
needed inductive reactance in step 90. Once the variable inductor
is tuned to provide the desired amount of inductance,
electrosurgery is continued at optimal impedance levels in step
92.
A variety of methods for identifying a capacitive reactance and
tuning a variable inductor can be utilized without departing from
the scope or spirit of the present invention. For example, an
electrode of the size and type that is typically utilized during
electrocardiogram procedures can be utilized with a separate
monitoring current to determine the capacitive reactance of the
return electrode before, during, or after the procedure. In another
embodiment, the variable inductor can be continually adjusted
during the course of a surgical procedure to provide an optimal
amount of inductance as the patient contact area and capacitive
reactance varies.
While the present invention is described above primarily with
reference to a series inductor for use with a capacitive
electrosurgical return electrode, a series capacitor can be
utilized with a self-limiting electrosurgical return electrode
having an inductive component without departing from the scope and
spirit of the present invention. In the embodiment, the series
capacitor provides a level of capacitive reactance needed to
counteract the inductive reactance of the electrosurgical return
electrode. A variety of type and configuration of the series
capacitors can be utilized without departing from the scope and
spirit of the present invention.
Contact Quality Monitoring
With reference now to FIG. 8, there is shown an electrosurgical
system 110 that utilizes one or more aspects of the present
invention. As depicted, system 100 includes an electrosurgical
return electrode 110 that communicates with an electrosurgical
power unit 130 via members 122 and 124. The electrosurgical power
unit 130 delivers electrosurgical signals or radio frequency (RF)
energy to an electrosurgical tool or instrument 140 that can be
used during a procedure to cut and/or coagulate tissue of a
patient.
The electrosurgical power unit 130 also includes contact quality
monitoring circuitry 134. In the illustrated embodiment, circuitry
134 creates a contact quality monitoring signal that is delivered
to electrosurgical return electrode 110 utilizing member 124. In
other configurations, the monitoring signal is deliverable along
members 122 and/or 124. This monitoring signal can have a variety
of different waveforms, frequencies, power levels, phase angle, or
combinations thereof to allow circuitry 134 to measure, sense,
and/or track the monitoring signal as it is delivered to and
received from electrosurgical return electrode 110 along the
monitoring path; the path extending from electrosurgical power unit
130, along member 124, through electrosurgical electrode 110 and a
patient (not shown), and returning to electrosurgical power unit
130 along member 122. Differences in power, waveform, frequency,
phase angle, or any other measurable characteristic or property of
the monitoring signal can be measured, sensed, and/or tracked to
identify whether a patient (not shown) is sufficiently in contact
with electrosurgical electrode 110 to prevent patent burns.
In addition to the above, it will be appreciated by those skilled
in the art that the monitoring signal and associated circuitry and
path can be configured to provide a variety of information relating
to the contact area between the patient and a return electrode of a
variety of types and complexities. For example in one embodiment of
the present invention, the monitoring circuitry can be configured
to simply determine when the contact area falls below a
predetermined threshold. In an alternative embodiment, the
monitoring circuitry can be configured to determine the actual
contact area and provide related information such as the amount of
electrosurgical current and/or current densities. In yet another
embodiment, the monitoring circuitry provides information needed to
tune a variable inductor so as to counteract capacitive reactance
in the electrosurgical circuit.
As shown, electrosurgical return electrode 110 electrically
communicates with electrosurgical power unit 130 through members
122 and 124. Return electrode 110 is adapted to prevent patient
burns by providing self-limiting capabilities and to cooperate with
circuitry 134 to determine whether the contact area between the
patient and return electrode 110 is below a given threshold.
Return electrode 110, in the exemplary embodiment, includes a
semi-insulating member 112 and a conductor member 114. In this
configuration, semi-insulating member 112 is adapted to provide the
self-limiting characteristics or capabilities of return electrode
110. Conductor member 114 is configured to permit contact quality
monitoring circuitry to determine the contact area between return
electrode 110, such as but not limited to semi-insulating member
112, and a patient resting thereon. In the illustrated embodiment,
conductor member 114 has a split-plate configuration with a first
conductor 111a and a second conductor 111b. Conductor member 114
need not be in direct physical contact with the patient. A patient
can be in electrical connection with first conductor 111a and
second conductor 111b without requiring the use of adhesives or
gels. This also allows return electrode 110 to be re-used thereby
eliminating the need and cost of disposable split-plate electrodes
that are currently used.
In the illustrated embodiment, a monitoring signal is passed to
conductor member 114, i.e. from first conductor 111a to second
conductor 111b. Members 122 and 124 operate to relay the monitoring
signal to and from contact quality monitoring circuit 134. At least
one of members 122 and 124 also operates as the return path of the
electrosurgical current. Where the contact area between the patient
and return electrode 110 is very low, the total effective impedance
on the monitoring signal or current will be very high and the
amount of monitoring signal or current will be minimized. Where the
contact area between the patient and electrode 110 is above the
minimum contact area, the total effective impedance will be much
lower allowing greater monitoring signal or current to flow.
By determining the amount of monitoring signal or current, the
contact quality monitoring circuitry 134 determines whether the
contact area between the patient and electrode 110, such as but not
limited to semi-insulating member 112, is above a predetermined
threshold (e.g. minimum contact area). Where the contact area is
below the predetermined threshold the monitoring circuitry 134
activates an output device, such as but not limited to, an output
device capable of delivering an audible signal, a visual signal, a
tactile signal, or a combination thereof, to notify a physician or
user that the contact area is insufficient to conduct effective
surgery. As will be appreciated by those skilled in the art, the
contact quality monitoring circuitry can be configured to determine
the amount of contact area between a patient and a return electrode
in a variety of manners utilizing an electrosurgical return
electrode having one or a combination of a resistive component, a
capacitive component, and/or an inductive component.
With reference now to FIG. 9-13 there is shown a variety of
configurations of a conducting member for use with circuitry 134
(FIG. 8) of electrosurgical power unit 130, in which the conducting
member allows circuitry 134 to determine whether the total contact
area between the patient and the return electrode is within a given
range or above a threshold level below which the patient receives a
burn. One benefit of the configuration of the conducting members of
FIGS. 9-13 is that they permit circuitry 134 to optionally
determine the amount of contact area between the patient and the
return electrode notwithstanding the total surface area of the
semi-insulating member and the portion of semi-insulating member
the patient is contacting. For the sake of simplicity, the
conducting members will be described for use with contact quality
monitoring circuitry, however as will be appreciated by those
skilled in the art a variety of types and configurations of
circuitry can be utilized within the scope and spirit of the
present invention to determine whether the contact area between the
patient and the semi-insulating member is below a given
threshold.
The configuration of the conducting members of FIGS. 9-13 is
particularly well suited for use with the semi-insulating member
112 of FIG. 8. Semi-insulating member 112 is configured to have a
sufficient surface area to permit a patient to contact various
portions of semi-insulating member 112 while maintaining a minimum
contact area. Traditional split-plate electrodes having two
independent conductive layers positioned side-by-side are not
configured to allow contact quality monitoring circuitry to
determine the amount of contact area independent of the location of
the patient on a return electrode. For example, traditional
split-plate electrodes are unable to identify that the patient
contact area is sufficient to conduct safe and effective
electrosurgery where the patient is contacting only one side of the
return electrode. The configurations of conductive members of FIGS.
9-13 allow contact quality monitoring circuitry 134 to determine
the amount of contact area between the patient and a return
electrode notwithstanding the total surface area of the electrode
and the portion of the electrode the patient is contacting. While
the conducting members of FIGS. 9-13 are particularly well adapted
for use with the semi-insulating member 112 of FIG. 8, it will be
understood that conducting members of FIGS. 9-13 can be utilized
with contact quality monitoring circuitry independently of a
semi-insulating member.
With reference now to FIG. 9, there is shown a conductor member 214
in which segments of a first conductor 222 and a second conductor
224 are arranged in a matrix. First conductor 222 includes segments
222a-222n, while second conductor 224 includes segments 224a-224n.
Segments 222a-222n are electrically isolated from segments
224a-224n such that a monitoring signal passes from first conductor
222 to second conductor 224 through the patient rather than
directly from segments of first conductor 222 to the segments of
second conductor 224.
Segments 222a-222n of first conductor 222 are electrically coupled
in parallel. Segments 224a-224n of second conductor 224 are also
electrically coupled in parallel. Because the segments are
electrically coupled in parallel, the impedance level varies based
upon the number of adjacent segments in contact with the patient.
The matrix configuration of segments 222a-222n and 224a-224n
permits contact quality monitoring circuitry to determine whether
the contact area between the patient and the return electrode is
sufficient to prevent patient burns or allow effective surgery
notwithstanding the total surface area of the semi-insulating
member and the portion of semi-insulating member the patient is
contacting.
While segments of first conductor 222 and second conductor 224 are
depicted as having a checkerboard configuration, it will be
understood that a variety of configurations of conductor member 214
are possible. For example, first conductor 222 and second conductor
224 can be arranged in alternating stripes, triangles, ellipses, or
any other configuration allowing the contact quality monitoring
circuitry to determine the amount of contact area between the
patient and the return electrode notwithstanding the total surface
area of the return electrode and/or semi-insulating member and the
portion of the return electrode and/or semi-insulating member the
patient is contacting.
FIG. 10 illustrates another alternate configuration of a conductor
member 314. As illustrated, conductor member 314 includes a first
conductor 322 and second conductor 324 that are interwoven in a
lattice structure. The segments 322a-322n and 324a-324n are
electrically coupled in parallel. Additionally, first and second
conductors 322 and 324, respectively, are electrically isolated
from one another. The interwoven lattice structure permits the
segments to alternate while providing a configuration that allows
for efficient and convenient manufacture of conductor 314.
FIGS. 11A and 11B illustrate a first conductor 422 and second
conductor 424 that are configured to form conductor member 414
having a split-plate type configuration. In the illustrated
embodiment first conductor 422 includes a plurality of segments
422a-422n. Segments 422a-422n are defined by a plurality of voids
426a-426n. Similarly, second conductor 424 includes a plurality of
segments 424a-424n and a plurality of voids 428a-428n. First
conductor 422 and second conductor 424 can be manufactured by
stamping a sheet of conductive material to create the segments and
voids, or by any other acceptable manufacturing process. The
segments and voids of first conductor 422 are configured to be out
of alignment with the segments and voids of second conductor 424
such that when the first conductor 422 is placed over the second
conductor 424 a matrix analogous to that shown in FIG. 9 is
created.
As will be appreciated by those skilled in the art, the
configuration of the conductor member is not limited to that shown
in FIGS. 9-11. A variety of configurations of a conductor member
can be utilized which allow the conductor member to be utilized
with contact quality monitoring circuitry to determine the amount
of contact area between the patient and the return electrode
notwithstanding the total surface area of the semi-insulating
member and the portion of the electrode and/or the semi-insulating
member the patient is contacting. For example, a first conductor
having a plurality of apertures formed therethrough can be placed
in electrical isolation over a second continuous sheet conductor
such that when a patient is positioned over a portion of the return
electrode a monitoring signal can pass from the first conductor to
the second conductor through the apertures.
With reference now to FIG. 12, there is shown a conductor member
514 having a plurality of membrane switches 522a-522n. The membrane
switches electronically communicate with contact quality monitoring
circuitry to receive a monitoring signal and return all or a
portion of the signal to circuitry. In the illustrated embodiment,
the plurality of membrane switches 522a-522n are adapted to permit
circuitry to determine whether the contact area between the patient
and return electrode is below a given threshold or threshold level
below which the patient receives a burn. The configuration of
membrane switches 522a-522n allows the contact area to be
determined notwithstanding the total surface area of the return
electrode and the portion of the return electrode the patient is
touching. A variety of mechanisms can be utilized to determine the
number of membrane switches depressed including, but not limited
to, software, digital circuits, and the like.
As will be appreciated by those skilled in the art, while conductor
member 514 is depicted as having a plurality of membrane switches
522a-522n, a variety of mechanisms can be used in the place of
membrane switches without departing from the scope and spirit of
the present invention. For example, an alternative electrical,
mechanical, electromechanical, and/or any other mechanism can be
used with conductor member 514 to indicate the amount contact area
between the patient the return electrode such that a contact
quality monitoring circuit can determine the amount of contact area
between the patient and the return electrode notwithstanding the
total surface area of the semi-insulating member and the portion of
semi-insulating member the patient is contacting.
FIG. 13 illustrates exemplary components of a membrane switch 522
that can be utilized in connection with the conductor member 514 of
FIG. 12. In the illustrated embodiment, membrane switch 522
includes a membrane layer 584, a tactile layer 586, a static layer
588, and a rigid layer 589. The membrane layer 584 includes a first
conductor adapted to receive a monitoring signal or current from
contact quality monitoring circuitry and is configured to be
deformed in response to a force acting thereon. The tactile layer
586 includes a dome member and is configured to separate the
membrane layer 584 from electrically coupling to static layer 588
until a force is applied to tactile layer 586 to deform tactile
layer 586 so that tactile layer 586 comes into contact with static
layer 588.
Static layer 588 comprises a second conductor configured to receive
the monitoring signal or current from membrane layer 584 when
membrane layer 584 and the tactile layer 586 are deformed. The
static layer 588 is electrically coupled to contact quality
monitoring circuitry to complete the monitoring path and allow
circuitry 134 (FIG. 2) to determine the contact area between the
patient and the return electrode.
The rigid layer 589 is configured to provide a substrate to prevent
deformation of static layer 588 and maintain electrical coupling
between membrane layer 584 and the static layer 588 when tactile
layer 586 is deformed. In the illustrative embodiment, membrane
layer 584 of each membrane switch is electrically coupled in
parallel with the membrane layers of all the other membrane
switches while the static layer 588 of each membrane switch is
electrically coupled in parallel with the membrane layers of all
other membrane switches.
As will be appreciated by those skilled in the art, a variety of
types and configurations of membrane switches can be utilized
without departing from the scope or spirit of the present
invention. For example, in one embodiment a single static layer
comprising a first conductor is positioned to be in contact with a
plurality of membrane layers comprising a plurality of second
conductors such that when a user is in contact with the surgical
surface of the return electrode a monitoring signal can pass
between the first conductor and each of the second conductors
positioned in the portion of the return electrode in contact with
the patient. The properties of the monitoring signal vary with the
number of second elements passing a monitoring signal to the first
element. The properties of the monitoring signal represent the
amount of contact area between the patient and the electrosurgical
surface.
Although the invention hereof has been described by way of
preferred embodiments, it will be evident that adaptations and
modifications may be employed without departing from the spirit and
scope thereof.
The terms and expressions employed herein have been used as terms
of description and not of limitation; and, thus, there is no intent
of excluding equivalents, but, on the contrary, it is intended to
cover any and all equivalents that may be employed without
departing from the spirit and scope of the invention.
* * * * *